Loading coils for radio antennae, and particularly for citizen's band radios, must have an impedance which is balanced with other parts of the radio circuit in order to obtain satisfactory results. In the manufacture of radios, it is highly desirable to have uniformity in the loading coils, so that each radio does not have to be independently tuned in order to match its loading coil. It is an object of the present invention to provide a method of making loading coils that have uniform characteristics, especially impedance.
In addition to having the same number of turns of wire on cores of the same size, it is important to have the convolutions of the coil uniform in their positions adjacent to one another. Even when coils are wound with great uniformity, however, the uniformity can be destroyed by displacement of convolutions during transport of the coil from one operation to another or by the flow of insulating material into the mold cavity of an injection molding machine.
In accordance with the method of this invention, wire coils are wound around non-metallic cores with connectors at opposite ends of the cores and confronting sides of the connectors forming flanges at opposite ends of a bare section of the non-metallic core that extends between the connectors. The wire is carefully wrapped on the core at a location between the flanges and spaced from each flange and with an end portion of the wire attached to a corresponding connector.
Before moving the coil or wire from the location at which it has been wound, a short length of adhesive tape is applied over the wire with the tape long enough to extend around the circumference of the coil and slightly further so as to provide an overlap seam for the tape. The tape is of a width slightly greater than the axial length of the coil, so that the tape not only surrounds the full circumference of the coil but also extends for a short distance beyond both ends of the coil. Pressure is then applied to the tape while the core rotates so as to put the end portions of the tape, beyond the coil, in adhering contact with the core. This holds the wire of the coil against any movement while the assembly is transported to the next operation in the manufacture of the loading coil.
The tape-wrapped coil is placed in a cavity of molding apparatus and insulating material is introduced into the cavity to form a jacket around the coil and the portions of the core beyond the ends of the coil. The adhesive tape, which is preferably a polyester insulating tape, is left on the coil and is a permanent part of the coil after the insulation has been applied over the tape. Even when using high pressure injection molding machines, the rush of molten material into the cavity cannot displace or alter the relationship of the convolutions of the coil to one another, because they are held in place by the tape and the molten material does not ever have direct contact with any of the convolutions.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.